1. Field of the Invention
The present invention relates to a laser diode which is used in optical information processing, optical communication and the like. More particularly, it relates to a ridge type laser diode and a method for producing the same.
2. Prior Art
At present, in a laser diode, a ridge type laser diode enclosing light in a transverse direction (width direction) of an active layer is used for various purposes. FIG. 14A is a schematic view showing a construction of a conventional ridge type laser diode. The ridge type laser diode shown in FIG. 14A is made by forming an n-type lower cladding layer 2 of n-type Al.sub.0.5 Ga.sub.0.5 As an active layer 3 of a quantum well structure, a p-type first upper cladding layer 4 of p-type Al.sub.0.5 Ga.sub.0.5 As, a ridge waveguide 11, a p-type contact layer 9 of p-type GaAs, a p side electrode 14 and an insulating film 12 on the upper surface of an n-type semiconductor substrate 1 of n-type GaAs, wherein an n side electrode 15 is formed on the lower surface.
This conventional ridge type laser diode is produced by the following method for production. That is, as shown in FIG. 13A, the n-type lower cladding layer 2, the active layer 3 having a quantum well structure, the p-type first upper cladding layer 4 and the p-type contact layer 9 are respectively formed on the upper surface of the n-type semiconductor substrate 1 by epitaxial growth. After an insulating film is formed on the surface (whole surface of a wafer) of the p-type contact layer 9, the insulating film is patterned into a stripe-like configuration to form a stripe-like insulating film 10 as shown in FIG. 13B. As a material of this insulating film 10, Si.sub.2 N.sub.4, SiO.sub.2, etc. is used. Then, the p-type contact layer 9 and the p-type upper cladding layer 4 are etched to the midst of the p-type upper cladding layer 4 by using the insulating film 1 formed in the stripe-like configuration as a mask (ridge etching). As a consequence, the ridge waveguide 11 is formed as shown in FIG. 13C. When the p-type upper cladding layer 4 is formed of p-type Al.sub.0.5 Ga.sub.0.5 As and the p-type contact layer 9 is formed of p-type GaAs, examples of an etchant for the ridge etching include a mixture of tartaric acid and hydrogen peroxide or of sulfuric acid, hydrogen peroxide and water.
After the ridge etching is conducted and the stripe-like insulating film 10 is removed by wet or dry etching, an insulating film 12 is formed again on the whole surface of a wafer, as shown in FIG. 13D. Then, by using a photolithography technique, the insulating film 12 is removed only at a planar portion on the upper part of the ridge 11 to provide an opening part in the insulating film 12 by means of dry etching. When the p side electrode 14 is formed from above the opening part, the p side electrode 14 comes into contact with a crystalline layer only at the opening part on the upper surface of the ridge 11 with the result that current flows only through this part. Lastly, an n side electrode 15 is formed on the lower surface of the n-type semiconductor substrate 1 and a laser is separated from the wafer by means of cleavage to form a reflecting surface, thereby completing the device shown in FIG. 14A.
Next, an operation of a conventional ridge type laser diode shown in FIG. 14A will be explained.
In the ridge type laser diode, when a voltage is applied so that the p side electrode 14 becomes positive and the n side electrode 15 becomes negative, holes are injected into the active layer 3 having the quantum well structure through the p-type contact layer 9 and the p-type upper cladding layer 4, while electrons are injected into the active layer 3 through the n-type semiconductor substrate 1 and the n-type lower cladding layer 2. The holes and electrons recombine in an active region of the active layer 3 to emit light. At this time, when the light which exceeds the loss of the waveguide to a sufficiently high level, laser oscillation is generated. In case of the laser oscillation, a region other than the ridge waveguide 11 is covered with the insulating film 12 so that a current does not flow in regions other than the ridge waveguide. That is, the current flows only through the ridge waveguide 11 so that the laser oscillation is generated in the active layer 3 at the lower part of the ridge waveguide 11.
Generally, in the laser diode, the laser beams are enclosed in the active region with a difference in refractive index between the active layer and the cladding layer in a vertical direction with respect to the substrate. In the laser diode, the light in the vertical direction is effectively enclosed over the whole waveguide. On the other hand, in a horizontal direction with respect to the substrate, in the case of the ridge type laser diode, the light is enclosed in a high refractive index region having a high refractive index immediately below the ridge waveguide 11 outside of the active layer 3 and the light is guided by a difference in effective refractive index between the region immediately below the ridge waveguide 11 in the active layer and other regions. A refractive index distribution in a horizontal direction in the active layer 3 is shown in FIG. 14B. In order to obtain a stable laser oscillation in the ridge type laser diode, it is necessary to stabilize a horizontal transverse mode. In order to stabilize this horizontal transverse mode, it is necessary to set the width of the ridge waveguide 11 to be narrow so that a higher mode is not generated at the time of guiding the light in the high refractive index region. The width of the ridge waveguide 11 is preferably set to not more than 3 .mu.m.
However, the thickness of the cladding layer is generally required to be set to not less than 1.5 .mu.m in order to effectively enclose the laser beams generated in the active layer within the active layer. When an attempt is made to form the ridge waveguide 11 having a width of not more than 3 .mu.m and a thickness of not less than 1.5 .mu.m, the surface of the upper part of the ridge waveguide 11 becomes not more than 1 .mu.m, and almost no planar part is present.. Consequently, it becomes extremely difficult to transfer at the time of removing the insulating film 12 on the upper surface of the ridge waveguide 11. Actually, it is impossible to form the ridge waveguide having a width of not more than 3 .mu.m. Accordingly, it was impossible to prevent a higher mode from being generated and to stabilize the horizontal transverse mode by reducing the width of the ridge waveguide.
Referring to the refractive index distribution shown in FIG. 14B and the generation of the higher mode in the case where the light is guided in the high refractive index region of the active layer 3, the higher propagation mode is easily generated with the increase of a difference in refractive index between the high refractive index region and other parts. Accordingly, when the difference in refractive index is reduced, the width of the ridge waveguide 11 can be made wider. However, in this case, when current injection is increased, the refractive index of the central part of the high refractive index region having a high current density decreases as shown in FIG. 14C, thereby causing a phenomenon wherein the laser beams vary with a slight fluctuation of the current distribution. As a consequence, there arose a problem of generating a kink as a non-linear part where the light output does not increase in proportion to current in the practical range of the light output-current characteristics, thereby causing grave trouble in a practical use thereof. Accordingly, it has hitherto been difficult to obtain a ridge type laser diode which can stabilize the horizontal transverse mode and cause little peak output variation.